1. Field of the Invention
This invention relates to a method and apparatus for joining together shaped bodies of polytetrafluoroethylene by a butt joint.
2. Description of the Prior Art
Shaped parts which consist of polytetrafluoroethylene (PTFE) or contain predominantly PTFE hereinafter designated polytetrafluoroethylene material are increasingly used as corrosion-resistant linings, as for example, for chemical apparatus such as heat exchangers, columns and vessels, because of their excellent thermal and chemical stability. In this connection, since fabrication of PTFE tubes of large diameter is not possible by ram or paste extrusion, it is necessary, particularly for equipment with a large inside diameter, to join together shaped parts, e.g., plates or foil to make them gas-and liquid-tight. Further difficulties are due to the fact that a separate press orifice is necessary for every diameter, and the ratio of wall thickness to diameter can be varied only within certain limits.
It has also not been possible to date to make cemented joints with a quality required, for example, for chemical apparatus, particularly with high thermal and chemical stability, because of the poor wettability of polytetrafluoroethylene and the comparatively low thermal and chemical stability of adhesives. Even if the PTFE surfaces to be joined together are first etched, the quality of the joint is not sufficient for the application mentioned. The welding methods commonly used for joining together conventional plastic materials, in which the pressed-together parts are heated above the melting or softening point, are not usable for polytetrafluoroethylene. However, overlapping joints between thin PTFE foils have been made by this method, but are not well suited for tank linings because of the extra material thickness at the overlap. The sealing of such seams, for example, as flanges, is particularly difficult and expensive if thicker foil or plates are used. Attempts to avoid such overlap points have been made according to the French Patent Application 2,163,420 by a method for making a joint between PTFE parts by a butt joint, in which the V-shaped seam gap between two PTFE plates fixed on a support is filled with a PTFE powder and the latter is heated by heating elements of stamp-like design, which are pressed on the joint. Sufficient and reproducible welding quality which depends, among other factors, on the quality and grain of the powder as well as on the packing density in a complicated manner, can be achieved by this process only if all the factors influencing the operation are obtained, which is hardly possible under normal operating conditions. The asymmetrical temperature distribution in the material to be welded, caused by the arrangement of the support underneath the weld results in unfavorable welding quality.